In current telecommunication systems, digital information of interest is typically communicated from a transmitter at one location to a receiver at another location by first forming a sequence of symbols based on the digital information and then using the symbol sequence to modulate a single carrier signal or a multiple carrier signal. At the receiver, the carrier signal is removed and the resultant, so called, ‘baseband’ signal is processed to recover first the symbols and then the digital information of interest. In general, signals used to communicate digital information from a transmitter to a receiver can be referred to as digital communication signals. Although the details of the mapping of the digital information onto the symbols vary from one application to another as do the details of the signal modulation, it is standard practice in the design of digital communication signals to use a fixed symbol rate (or a well defined set of fixed symbol rates) such that the individual symbols are used to modulate the signal for a fixed interval of time. The inverse of this individual symbol time interval is referred to as the symbol rate.
It is also standard practice in the design of digital communication signals to place what is referred to as a pulse shaping filter in the transmitter-to-receiver channel response or equivalently, the transmitter-to-receiver transfer function. The pulse shaping filter imposes a shape to the individual symbol ‘pulses’ so as to minimize the interference between the symbol pulses at the communications signal receiver. By far the most common example of a pulse shaping filter is the raised cosine filter (RCF). Typically the RCF is distributed between the transmitter and the receiver such that a root raised cosine filter (RRCF) is at both the transmitter and the receiver, the net contribution to the transmitter-to-receiver channel response being equivalent to one RCF.
It is also standard practice in the design of digital communication signals to impose a framing structure on the sequence of symbols that is used to modulate the single carrier or multiple carrier signal. Once the framing structure is properly identified it is an aid to the communications signal receiver in that it simplifies the process of extracting the digital information of interest. An example of such a framing structure is the ‘slot’ structure in the Wideband Code Division Multiple Access (WCDMA) signal specified by the Third Generation Partnership Project (3GPP) standards organization. The WCDMA slot consists of 10 sub-frames of 256 chips each, the symbols being related to the chips by a spread code. For the WCDMA signal and code division multiple access signals in general, the fundamental timing interval is the chip rate whereas the symbol rates are well defined multiples of the chip rate.
Symbol (chip) timing recovery refers to the process in the communications signal receiver that estimates the time when the information and/or energy associated with individual symbols (chips) arrives in the received communications signal. Since the transmitter typically clocks the symbol (chip) interval based on a crystal oscillator, in order to be accurate the timing recovery process at the receiver must be capable of dynamically tracking changes in the fundamental timing interval that are due to variations in the transmitter's crystal oscillator frequency.
Frame timing recovery refers to the process in the communications signal receiver that estimates when the start and stop of each frame or sub-frame occurs in the received communications signal.
If the communications signal receiver is battery operated it is desirable to process the communications signal at a low sample rate in order to reduce the computations per symbol (digital information of interest). Fewer computations per symbol result in lower power consumption by the receiver and extend the battery life. This is especially desirable for today's mobile broadband communication devices, examples being third generation (3G) mobile phones and battery operated computers with embedded wireless broadband network interfaces.
Symbol timing recovery for the above described digital communications signals is an important function. However, traditional timing recovery schemes require an over-sampling of the communications signal, e.g., a sampling at a rate of 2/T or 4/T, resulting in various inefficiencies. As such, what is needed is a system and method that overcomes this limitation.